The central panel shows the χ² landscape (up to values of 100) in the ζ versus t plane for the P2 position for the XR search (i.e. with n_H = 1.0 × 10⁵ cm⁻³, A_v = 20 mag and T = 15 K) Case 2 (fitting to abundances of all species). The location of the minimum χ² is marked with a black cross. The surrounding eight panels show the effect on the χ² landscape of stepping in the model grid to the next higher and lower values of density and temperature (as labelled). We note that A_v is held fixed at 20 mag in all these panels (so these are just a subset of the UR search).